Ivermectin is a broad spectrum
antiparasitic drug introduced onto the international animal health
market in 1981. In Britain it is approved for the use in cattle,
sheep, goats, pigs and horses. The drug is absorbed systemically
after administration and is excreted mainly in the faeces. Being
insecticidal, residues of ivermectin in cow dung can reduce the
number and variety of insects in the dung beetles, residues of the
drug have been shown to adversely affect certain types of fly
larvae, appearing to inhibit larval development and/or prevent
pupation from taking place.

Fears have been expressed that this
reduction in the fauna could have an adverse effect on the
wildlife, particularly birds, reliant on cow dung as an important
invertebrate food source. Such a reduction also has the potential
to delay dung degradation, leading to fouling of pastures. In
addition, there is concern over the effects that long-term
ivermectin usage in any area may be having on the populations of
insects associated with cow dung within that area. The purpose of
this information note is to provide a summary of the currently
available chemicals for treatment of cattle against internal
nematode parasites and certain arthropod ectoparasites, that are
more 'environmentally sensitive' than ivermectin (Ivomex as an
injection or pour-on formulation).

ANTHELMINTIC TREATMENT

The figure below shows the major chemical
groupings available for anthelmintic treatment of stock, and the
spectrum of activity of each. No adverse effects on the
invertebrate fauna of cow dung have been reported after treatment
of cattle with chemicals from the first three of these groupings,
and therefore they provide viable alternatives to ivermectin.

Benzimidazoles

Most of the anthelminitics currently available
belong to a group of chemicals commonly referred to as
benzimidazoles. These are highly effective against most of the
gastrointestinal nematodes of cattle, and the ranges of activity of
the more recently introduced benzimidazoles include lungworms,
tapeworms and liver fluke. These chemicals have, with one
exception, to be given by mouth and they are marketed in a range of
drench and paste preparations. Oxfendazole is also available for
intra-ruminal injection or incorporated into one of two
pulse-release devices ('Autoworm' or 'Synanthic Multiclose') which
are normally administered at the start of the grazing season. A
does of oxfendazole is released from the devices at regular
intervals throughout the grazing season, up to a maximum of 5 or 6
times, giving them active lives of up to 130 days.

Imidazothiazoles

Levamisole is highly effective against a range of
gastointestinal nematodes and lungworms parasitising cattle. While
it is normally given by mouth or subcutaneous injection, pour-on
preparations are also marketed for use in cattle. Levamisole
preparations should not be given simultaneously with
organophosphorous preparations, or for a period of 14 days before
or after treatment.

Tetrahydropyrimidines

Morantel tartrate is highly effective against
adult and developing immature gastrointestinal nematodes, and is
currently marketed as a slow-release device in cattle ('Paratext
Flex Bolus') which remains active for about 90 days. Morantel has
no activity against lungworms, and therefore a lungworm vaccination
programme should be used in conjunction with this chemical.

SPECTRUM OF ACTIVITY OF ANTHELMINTIC GROUPS

ECTOPARASITIC TREATMENT

Ivermectin is also approved for use against certain
ectoparasites of cattle (eg sucking lice, warble fly), and so
alternative control methods for these pests need to be implemented
if ivermectin usage is discontinued. Two main chemical groupings
are used as the basis for the commonly used ectoparasitivides,
namely the organophosphates and the synthetic pyrethroids.

Organophosphates

These chemicals can persist in the animals coat
for a reasonable period, but residues in animal tissues are
short-lived. Some organophosphates, eg coumaphos, are formulated as
powders for use against lice and fleas, while others, eg
chlorphyrifos, are used as sprays for tick control. Some have the
ability to act systemically, given orally or as a pour-on, and can
be used for the control of warble fly larvae, lice and the mites
causing sarcoptic and chorioptic mange of cattle. These chemicals
can be dangerous for the operator to use, therefore, protective
clothing should be worn when applying them.

It should be noted that residues of the organophosphate
dichlorvois in horse dung (from animals treated with the drug to
control internal nematode and arthropod parasites) can have an
adverse effect on the beetle fauna of the dung, and therefore the
use of systemic members of this grouping should be considered
carefully. In addition, magpies have died after feeding on the
backs of cattle treated with organophosphate warblicides.

Synthetic pyrethroids

These persist well on the coat or skin, but not
in tissue, and so are of particular value against parasites which
feed on the skin surface, eg. Lice, ticks on sheep, some mites and
nuisance flies. They have a low mammalian toxicity, but they are
very poisonous to fish and crustaceans (and so containers should be
disposed of with care). Synthetic pyrethroids can be applied to
cattle as sprays for fly and lice control; impregnated in ear tags
for fly control; or in a pour-on formulation for the control of
flies, lice and ticks.

ADDITIONAL COMMENTS

Whichever chemical and method of application is
selected, it is important to read the manufacturers instructions,
with particular regard to the types of pest against which the
product is active, the class of stock for which it is recommended
and any limitations to use, the dose rate, and the withholding
period. In addition, it is advisable to consult a veterinary
surgeon to develop a control strategy designed for the farm which
preferably does not rely too heavily on chemical treatment of the
stock.

It should also be borne in mind that the use of
alternative chemicals may result in an increase in antiparasitic
treatment costs, and may also necessarily involve more handling of
the animals throughout the year.

Finally, a note of caution – resistance of
certain nematodes to anthelmintics have been reported in sheep, and
therefore, although such resistance has not yet been reported in
cattle, it might be prudent to select a different drug from each
anthelmintic grouping on an annual basis to utilize their different
modes of action and thus minimize the development of
resistance.